Add type annotations to trace decorator

synapse/rest/client/room_keys.py

@@ -13,7 +13,7 @@
 # limitations under the License.
 
 import logging
-from typing import TYPE_CHECKING, Optional, Tuple
+from typing import TYPE_CHECKING, Optional, Tuple, cast
 
 from synapse.api.errors import Codes, NotFoundError, SynapseError
 from synapse.http.server import HttpServer
@@ -196,8 +196,11 @@ async def on_GET(
         user_id = requester.user.to_string()
         version = parse_string(request, "version", required=True)
 
-        room_keys = await self.e2e_room_keys_handler.get_room_keys(
-            user_id, version, room_id, session_id
+        room_keys = cast(
+            JsonDict,
+            await self.e2e_room_keys_handler.get_room_keys(
+                user_id, version, room_id, session_id
+            ),
         )
 
         # Convert room_keys to the right format to return.